That’s not terrible - My Windows machine runs two monitors at 2560x1440, and even with your file loaded and stretched across both displays, I max out around 13% usage in LightBurn.
It’s possible this is an interaction with Big Sur and the framework we use. There was an issue a few years back with MacOS doing gamma correction on the image being rendered to and presented to the OS. I’ll have to get Big Sur running on a machine to see if I can reproduce this.